We are all children of our time, so I guess that a “pure” Mozart cadenza is not really possible. It is odd, though, that so many of the “standard” Mozart cadenzas are composed in the 19th century. A 20th century musician playing a 19th century cadenza to an 18th century concerto sounds a bit too eclectic, [but might still have its own logic ;-)]. I would like to support numerous encouragements for a cadenza of your own. I think that if it’s your own, you’ll be OK, the audience will believe you–period style or not. Sometimes–when I feel like being provocative–I say that all cadenzas are bad, except for the composer/interpreter. [I used to love to listen to Nicolet do his own Mozart cadenzas, but they don’t work for me.] Personally, I try to emulate period style, but I have heard “modern” cadenzas that have worked very well.
My tips for making your own cadenza:
1. Make a plan (harmonic progression, dramatic development)
2. Use technical elements that suit you and the work in question
3. Don’t overquote the concerto (we don’t really need to hear those themes all over again).
4. Try to surprise and tickle the audience, the goal of the cadenza is suspense.
5. If you want to copy period style, analyze already existing cadenzas from that time.
6. If you feel insecure, find somebody with experience to help you, but make sure you bring in your own ideas.
7. Keep it short, don’t repeat yourself.